Magnetic assemblies and methods for producing optical effect layers comprising oriented platelet-shaped magnetic or magnetizable pigment particles

Abstract

The invention relates to the field of the protection of security documents such as for example banknotes and identity documents against counterfeit and illegal reproduction. In particular, the present invention provides magnetic assemblies and methods for producing optical effect layers (OELs) on a substrate, said method comprising a step of exposing a coating composition comprising platelet-shaped magnetic or magnetisable pigment particles to the magnetic field of the magnetic assembly so as to bi-axially orient at least a part of the platelet-shaped magnetic or magnetisable pigment particles.

The invention claimed is: 
     
         1 . A magnetic assembly for producing an optical effect layer on a substrate, said magnetic assembly being configured for receiving the substrate in an orientation that deviates not more than 10° from parallel alignment to a first plane and above the first plane, and further comprising:
 a) at least a first set and a second set, each of the first and second sets comprising:
 i. one first bar dipole magnet
 having a first thickness (L 1 ), a first length (L 4 ) and a first width (L 5 ), and 
 having its magnetic axis oriented in a position that does not deviate than 10° from parallel alignment to the first plane, 
 
 ii. two second bar dipole magnets
 having a second thickness (L 2 ), a second length (L 6 ) and a second width (L 7 ), 
 each of the two second bar dipole magnets having an upmost surface, wherein the upmost surfaces of the two second bar dipole magnets are flush with each other, and 
 magnetic axes of each of two second bar dipole magnets are oriented in a position that does not deviate than 10° from perpendicular alignment to the first plane, 
 the first plane being located above the upmost surface of the two second bar dipole magnets 
 
 
 the first bar dipole magnet of the first set having a magnetic direction opposite to the magnetic direction of the first bar dipole magnet of the second set, 
 the first bar dipole magnets of the first and second sets being spaced apart by a first distance (d 1 ), 
 the first length (L 4 ) and first width (L 5 ) of the first bar dipole magnet of the first set being within ±5% of the first length (L 4 ) and first width (L 5 ) of the first bar dipole magnet of the second set, respectively, and 
 the second lengths (L 6 ) and second widths (L 7 ) of the two second bar dipole magnets of the first set being within ±5% of the second lengths (L 6 ) and second widths (L 7 ) of the two second bar dipole magnets of the second set, respectively, 
 the first bar dipole magnet and the second bar dipole magnets of each of the first and second sets being aligned to form a column, in that the first bar dipole magnet of the first and second sets is respectively placed between and spaced apart from the second bar dipole magnets by a second distance (d 2 ), 
 a value of the first width (L 5 ) being within ±5% of a value of the second length (L 6 ), 
 a North pole of one second bar dipole magnet of each of the first and second sets pointing towards the first plane and a North Pole of the first bar dipole magnet pointing towards said one second bar dipole magnet of each of the first and second sets, and a South pole of another of the second bar dipole magnet of each of the first and second sets pointing towards the first plane and a South Pole of the first bar dipole magnet pointing towards said another second bar dipole magnet of each of the first and second sets, and further comprising: 
 b) a first pair (P 1 ) of third bar dipole magnets 
 having a third thickness (L 3 ), a third length (L 8 ) and a third width (L 9 ) and 
 magnetic axes of the first pair of third bar dipole magnets are oriented in a position that does not deviate than 10° from parallel alignment to the first plane, 
 the second width (L 7 ) of the two second bar dipole magnets of the first and second sets being within ±5% of the third width (L 9 ) of the third bar dipole magnets, 
 each of the third bar dipole magnets being aligned with one second bar dipole magnet of the first set and one second bar dipole magnet of the second set so as to form two lines, the third bar dipole magnets being placed between and spaced apart from the respective second bar dipole magnets by a third distance (d 3 ), 
 North poles of the third bar dipole magnets respectively pointing towards one of the second bar dipole magnets having North Poles pointing towards the first plane or South poles of the third bar dipole magnets respectively pointing towards one of the second bar dipole magnets having South Poles pointing towards the first plane, 
 wherein the first bar dipole magnets of the first and second sets, the second bar dipole magnets of the first and second sets, and the third bar dipole magnets are at least partially embedded in a non-magnetic supporting matrix. 
 
     
     
         2 . The magnetic assembly according to  claim 1 , wherein
 the first thickness (L 1 ) of the first bar dipole magnets of the first and second sets is equal to or smaller than the second thickness (L 2 ) of the second bar dipole magnets of the first and second sets;   the first thickness (L 1 ) of the first bar dipole magnets of the first and second sets is equal to or smaller than the third thickness (L 3 ) of the third bar dipole magnets of the first pair (P 1 );   wherein the second distance (d 2 ) between the first bar dipole magnet and the second bar dipole magnets is larger than or equal to 0 and smaller than or equal to 12 of the first thickness (L 1 ) of the first bar dipole magnets (0≤d 2 ≤½ L 1 ); and   wherein the third distance (d 3 ) between the third bar dipole magnets of the first pair (P 1 ) and the second bar dipole magnets of the first and second sets is larger than or equal to 0 and smaller than or equal to ½ of the first thickness (L 1 ) of the first bar dipole magnets (0≤d 3 ≤½L 1 ).   
     
     
         3 . The magnetic assembly according to  claim 1 , wherein the upmost surface of the second bar dipole magnets are flush with the upmost surfaces of the third bar dipole magnets. 
     
     
         4 . The magnetic assembly according to  claim 1 , wherein the first distance (d 1 ) between the first bar dipole magnets of the first and second sets is greater than or equal to 15% of the first length (L 4 ) and smaller than or equal to 150% of the first length (L 4 )(0.15*L 4 ≤d 1 ≤1.5*L 4 ). 
     
     
         5 . The magnetic assembly according to  claim 1 , further comprising one or more combinations comprising:
 i) a (2+i)th set (S 2+i )(i=a whole number greater than 0) comprising:
 one further first bar dipole magnet having the first thickness (L 1 ), the first length (L 4 ) and the first width (L 5 ), and having its magnetic axis oriented in a position that does not deviate than 10° from parallel alignment to the first plane, and 
 two further second bar dipole magnets having the second thickness (L 2 ), the second length (L 6 ) and the second width (L 7 ), each of the two second bar dipole magnets having an upmost surface, wherein upmost surfaces of the two second bar dipole magnets are flush with each other, and magnetic axes of the two second bar dipole magnets are oriented in a position that does not deviate than 10° from perpendicular alignment to the first plane, 
 the first bar dipole magnet of the (2+i)th set (S 2+i ) having a magnetic direction opposite to the magnetic direction of the first bar dipole magnet of the (2+i−1)th set (S 2+i−1 ) 
 the first bar dipole magnets of the (2+i)th and (2+i−1)th sets (S 2+i , S 2+i−1 ) being spaced apart by the first distance (d 1 ), 
 the first bar dipole magnet of the (2+i)th set (S 2+i ) having the length (L 5 ) and width (L 4 ) within ±5% of the length (L 5 ) and width (L 4 ) of the first bar dipole magnet of the (2+i−1)th set (S 2+i−1 ), and 
 the two second bar dipole magnets of the (2+i)th set (S 2+i ) having lengths (L 6 ) and widths (L 7 ) within ±5% of lengths (L 6 ) and widths (L 7 ) of the two second bar dipole magnets of the (2+i−1)th set (S 2+i−1 ), respectively, 
 the first bar dipole magnet and the second bar dipole magnets being aligned to form a column, in that the first bar dipole magnet of the (2+i)th set (S 2+i ) is placed between and spaced apart from the second bar dipole magnets by the second distance (d 2 ), 
 the first and second lengths (L 4  and L 6 ) having a value within ±5% of each other, 
 the North pole of one of the second bar dipole magnets of the (2+i)th set (S 2+i ) pointing towards the first plane and the North Pole of the first bar dipole magnet pointing towards that second bar dipole magnet, and 
   ii) a (1+i)th pair (P i+1 ) of third bar dipole magnets having the third thickness (L 3 ), the third length (L 9 ) and the third width (L 8 ) and magnetic axes of the (1+i)th pair (P i+1 ) of third bar dipole magnets being oriented in a position that does not deviate than 10° from parallel alignment to the magnetic axes of the third bar dipole magnets of a (1+i−1)th pair (P 1+i−1 ),
 each of the third bar dipole magnets being aligned with one second bar dipole magnet of the (2+i)th set (S 2+i ) and one second bar dipole magnet of the (2+i−1)th set (S 2+i−1 ) so as to form two lines, the third bar dipole magnets being placed between and spaced apart from the respective second bar dipole magnets by the third distance (d 3 ), 
 the North poles of the third bar dipole magnets respectively pointing towards one of the second bar dipole magnets of the (2+i)th and (2+i−1)th sets (S 2+i , S 2+i−1 ) and the North Poles of said ones of the second bar dipole magnets pointing towards the first plane or the South poles of the third bar dipole magnets respectively pointing towards one of the second bar dipole magnets of the (2+i)th and (2+i−1)th sets (S 2+i , S 2+i−1 ) and the South Poles of said ones of the second bar dipole magnets pointing towards the first plane, 
   wherein the first bar dipole magnets of the (2+i)th set (S 2+i ), the second bar dipole magnets of the (2+i)th set (S 2+i ), and the third bar dipole magnets of the (1+i)th pair (P 1+i ) are at least partially embedded in the non-magnetic supporting matrix.   
     
     
         6 . A printing apparatus comprising the magnetic assembly according to  claim 1  being mounted in the vicinity of a transferring device. 
     
     
         7 . A method for producing an optical effect layer on a substrate comprising the steps of:
 i) applying on a substrate surface a radiation curable coating composition comprising platelet-shaped magnetic or magnetisable pigment particles, wherein an X-axis and a Y-axis define a plane of predominant extension of the particles, said radiation curable coating composition being in a first, liquid state so as to form a coating layer;   ii) exposing the coating layer to a magnetic field of the magnetic assembly recited in  claim 1  so as to bi-axially orient at least a part of the platelet-shaped magnetic or magnetisable pigment particles;   iii) at least partially curing the radiation curable coating composition of step ii) to a second, solid state so as to fix the platelet-shaped magnetic or magnetisable pigment particles in-their adopted positions and orientations.   
     
     
         8 . The method according to  claim 7 , further comprising a further step of exposing the coating layer to a magnetic field of a magnetic-field-generating device so as to re-orient at least a part of the platelet-shaped magnetic or magnetisable particles, said further step being carried out subsequently to step ii). 
     
     
         9 . The method according to  claim 8 , wherein a step of selectively at least partially curing one or more first areas of the coating layer of the radiation curable coating composition of step ii) is carried out so as to fix at least a part of the platelet-shaped magnetic or magnetisable particles in adopted positions and orientations, such that one or more second areas of the coating layer remain unexposed to irradiation, said step being carried out prior to, partially simultaneously with or subsequently to the step of further exposing the coating layer to the magnetic field of the magnetic-field-generating device. 
     
     
         10 . The method according to  claim 7 , wherein the coating layer is exposed, in a single step, to the interaction of magnetic fields of the magnetic assembly and a magnetic-field-generating device comprising one or more hard magnetic magnets, the magnetic-field-generating device being mounted on a rotating magnetic cylinder or being a moveable magnetic-field-generating device. 
     
     
         11 . The method according to  claim 7 , wherein the coating layer is exposed, in a single step, to the interaction of the magnetic fields of the magnetic assembly and one or more soft magnetic plates carrying one or more indicia in the form of voids and/or indentations and/or protrusions, said one or more soft magnetic plates being placed on a rotating magnetic cylinder or being placed on a moveable device below the substrate. 
     
     
         12 . The method according to  claim 7 , wherein a distance between the upmost surface of the first bar dipole magnets and the substrate is greater than 0 and smaller than or equal to about 20 mm. 
     
     
         13 . The method according to  claim 7 , wherein step iii) is carried out by UV-Vis light radiation curing. 
     
     
         14 . The method according to  claim 7 , wherein at least a part of the platelet-shaped magnetic or magnetisable particles is constituted by platelet-shaped optically variable magnetic or magnetisable pigment particles. 
     
     
         15 . The magnetic assembly according to  claim 2 , wherein a ratio of the second thickness (L 2 ) of the second bar dipole magnets of the first and second sets over the first thickness (L 1 ) of the first bar dipole magnets of the first and second sets (L 2 /L 1 ) is equal to or smaller than 3 and greater than or equal to 1 (1≤L 2 /L 1 ≤3), or wherein a ratio of the third thickness (L 3 ) of the third bar dipole magnets of the first pair over the first thickness (L 1 ) of the first bar dipole magnets of the first and second sets (L 3 /L 1 ) is equal to or smaller than 3 and greater than or equal to 1 (1≤L 3 /L 1 ≤3). 
     
     
         16 . The magnetic assembly according to  claim 4 , wherein the first distance (d 1 ) between the first bar dipole magnets of the first and second sets is greater than or equal to 25% of the first length (L 4 ) and smaller than or equal to 120% of the first length (L 4 ) (0.25*L 4 ≤d 1 ≤1.2*L 4 ). 
     
     
         17 . The magnetic assembly according to  claim 4 , wherein the first distance (d 1 ) between the first bar dipole magnets of the first and second sets is greater than or equal to 25% of the first length (L 4 ) and smaller than or equal to 80% of the first length (L 4 ) (0.25*L 4 ≤d 1 ≤0.8*L 4 ). 
     
     
         18 . The printing apparatus according to  claim 6 , wherein the transferring device is selected from the group consisting of chains, belts, cylinders and combinations thereof. 
     
     
         19 . The method according to  claim 12 , wherein the distance between the upmost surface of the first bar dipole magnets and the substrate is smaller than or equal to about 10 mm and greater than about 2 mm. 
     
     
         20 . The method according to  claim 14 , wherein the platelet-shaped optically variable magnetic or magnetisable pigment particles are selected from the group consisting of magnetic thin-film interference pigments, magnetic cholesteric liquid crystal pigments and mixtures thereof.